This invention relates generally to devices for pre-heating internal combustion engines and more particularly to heaters of the type wherein a casing having a heater element is mounted directly on the engine block.
In the past a number of proposals have been made for minimizing the various problems arising from exposure of motor vehicles to extreme cold. It is well known that vehicles which have been stored, parked or otherwise idle for a period of time are often difficult or impossible to start. A number of factors contribute to this problem. First, the ability of the fuel to be vaporized in the carburetor is greatly diminished. Also, water or water vapors in the fuel lines and carburetor tend to freeze, often creating a plug that can completely block fuel flow. In addition, the oil in the crankcase is considerably more viscous when cold, creating an additional load on the starter motor and electrical system of the vehicle. Moreover, the cranking capacity of a typical storage battery decreases considerably when subjected to unusually low temperatures. And, the reduction in battery voltage adversely affects the intensity of the spark produced by the ignition circuit.
One approach toward solving the problem is the use of a modular heater plug which is installed in a suitable access passage in the engine block. Generally the plug has a heater element protruding into the interior of the block so as to be capable of contact with the coolant therein. When the vehicle is to be stored or parked, the driver merely connects a power cord leading from the plug to a suitable 120 volt power source.
While such devices operated in a generally satisfactory manner, several drawbacks became apparent. First, with water cooled engines it was essential that the level of coolant in the block be sufficient to insure complete immersion of the heater element at all times. Due to the relatively high power associated with such heater devices, typically 400-600 watts, the element could cause permanent damage to itself if sufficient surrounding engine coolant was not available to draw off the heat as it was produced. Moreover, the power consumed was generally maintained constant at this relatively high level, irrespective of the ultimate steady state temperature reached by the block and coolant. That is, no compensation was made for variations in the ambient temperature. Accordingly, continuous application of high power to heater devices of this type over prolonged periods, for example over night or over a weekend, was considered to be both dangerous and wasteful of energy.
Attempts have been made to periodically interrupt the power supplied to a block heater, wherein a control module containing a bimetal switch was strapped onto or otherwise fastened to one of the hoses or lines extending between the block and radiator. This arrangement had several disadvantages, however. In particular, the temperature of the coolant in the hose was not truly representative of that in the engine block, because the vehicle's water pump was not operating while the vehicle was idle. It was considered that any convective flow of the coolant from the block was of little significance as regards monitoring block temperature. Moreover, the use of a bimetal switch had drawbacks in that it was required to control relatively heavy currents, typically on the order of 4 or 5 amperes. Arcing of the contacts occurred, causing eventual degeneration of the same and leading to erratic operation. The life of the switch was thus limited, especially in view of the multiple cycles which would occur over a period of several seasons. As the switch contacts deteriorated, the thermal response of the device changed. That is, it would open and close at temperatures which were different from those initially established during manufacture or calibration. Accordingly it is considered that such devices are of limited utility, especially in environments that are subject to extremes in temperature, as in the present instance where the ranges extend from well below zero, to several hundred degrees Fahrenheit.
In addition, with bimetal switches, spike-like current pulses were generated since the switching point usually did not coincide with a zero crossing of the a. c. wave. Such pulses are constituted of a wide band of frequencies, and caused periodic interference to radio and television reception in the immediate locality surrounding the device.